The present invention relates to a screening system for screening a flowing medium, which system is provided with a screen and a means for intermittent cleaning of the screen, which cleaning means is provided with a dump flushing apparatus comprising a tilting water storage container arranged at a higher elevation than the screen and intended to produce a surge of a flowing screen cleaning medium which impinges upon the substantially clean side of the screen and flows thereafter through the screen.
The invention relates also to a method of cleaning a screen in a waste water purification plant, which screen is intended to screen waste water which contains coarse solid particles, the screen being intermittently cleaned by directing a surge of a body of screen cleaning water against the side of the screen located downstream thereof relative to the direction of flow of the waste water.
Purification plants for waste water are basically designed and dimensioned to treat the sewage which flows to the plants in a more or less steady stream. During and after a rainfall, specifically a heavy rainfall, the amount of water flowing to such plants increases such that the combined flow of sewage water including storm water exceeds the handling capacity of the various water treating sections of such plant. In order to cope with this problem, waste water purification plants are provided with an overflow. The excess volume of water which cannot be handled by the water purification plant or sewage treatment plant flows over such overflow and is led directly into rivers, streams, lakes, and so forth. Such overflows are generally provided with a screen which may be a mesh-like structure or a grate-like structure having a number of parallel extending rods in order to prevent coarse solid particles from reaching e.g. the river. The contaminants of mainly the sewage water will obviously also settle on such screens in addition to the coarse particles which are screened out such that a periodical cleaning of the screens is necessary in order to prevent a clogging thereof.
Generally, such cleaning of the screens has been customarily carried out by scratching the deposits off or by performing a cleaning operation utilizing pressurized water led thereto by a hose. These cleaning procedures necessitate, however, often a dismounting of the clogged or partially clogged screen. Such is obviously tedious and time-consuming work, whereby the cleaning effect has often proven to be insufficient. Specifically when cleaning the screens by pressurized water, thixotropic deposits, e.g. sludge and sewage, which are of a jelly-like consistency keep clinging to the screen and merely move from the section thereof momentarily being cleaned to another section of such screen or such deposits may yield by flowing away along the screen and then again return to the original position. Often those deposits will change under the influence of the spray of pressurized water into a flowable state such that they will easily move along the screen and after returning to the original state they again will solidify. Furthermore, all matter which has been finally removed from the screen which has been dismounted must be disposed of in a separate working step.